1. Field of the Invention
The present invention relates to processes for reducing the level of acid gas components (e.g. sulfur oxides) from acid gases. In particular, the invention relates to use of semi-dry calcium silicate hydrates in the form of free-flowing powders for removal of sulfur oxides (SO.sub.2 /SO.sub.3) and/or other acid gases such as hydrogen chloride (HCl), hydrogen fluoride (HF) and hydrogen bromide (HBr) from flue gases resulting from combustion of solid/liquid fuel or waste.
2. Description of the Related Art
Coal represents one of the most bountiful sources of energy in the world today. For example, it has been estimated that the known coal reserves in the U.S. alone could supply sufficient energy for domestic consumption for several hundred years. Unfortunately much of this coal contains high levels of sulfur which, when the coal is burned, is released into the atmosphere, generally in the form of sulfur dioxide. One of the most serious environmental problems associated with such sulfur emissions is the generation of atmospheric sulfuric acid, resulting in so-called "acid rain."
Attempts at controlling sulfur dioxide emissions from coal burning plants have led to the development of a number of advanced systems and processes for flue gas desulfurization. Fluidized-bed combustion, furnace lime injection, and flue gas desulfurization are some of the examples. In these processes, limestone and/or lime has been used as a sorbent which forms primarily calcium sulfate at a temperature above 700.degree. C.
Fluidized bed combustion (FBC) and scrubbers for flue gas desulfurization (FGD) represent two of the more promising advanced processes for power generation. FBC relates to the combustion of coal with limestone particles as the bed material, and has received increasing attention as a promising and versatile technology for clean power generation. Equally promising has been FGD, wherein sulfur-reactive sorbents are employed to remove sulfur from flue gases prior to their venting into the atmosphere. In developing the technologies for FBC and FGD, a search for sorbents more effective than limestone and/or lime, especially ones which are amenable to recycle, has been a challenging task.
Flue gas desulfurization by the means of spray dryer absorber and bag filter or electrostatic precipitator has recently received much attention. In the spray dryer/bag filter system, flue gas is contacted with a fine spray of an aqueous solution or slurry of a reactive alkali (typically lime), with SO.sub.2 removal and drying occurring simultaneously. The sulfur dioxide is absorbed into the water droplet during the constant rate period of drying until it shrinks to the extent that the particles touch each other. During the following falling rate period, the remaining water diffuses through the pores of agglomerated particles until the solids establish pseudo-equilibrium with the humid environment of spray dryer.
The third stage of drying may be called the second-falling rate period. Any drying/mass transfer during this period is limited by the diffusion of moisture from within tightly packed particles. The first two stages take place exclusively in the spray dryer. The majority of pseudoequilibrium period occurs in the duct joining spray dryer and bag filter and in the bag filter itself. Since not all moisture is removed from the solids in the spray dryer, the remaining moisture promotes further removal of SO.sub.2 in the bag filter. Therefore the total SO.sub.2 removal in the system is a sum of removal in the spray dryer and bag filter.
The recycle of product solids is among the options that have been tested to increase the utilization of reagent. Numerous reports indicate that recycle of product solids and fly ash results in substantial improvement of reagent utilization and SO.sub.2 removal. This option provides a higher Ca(OH).sub.2 concentration in the slurry feed at the same Ca(OH).sub.2 stoichiometry (moles of Ca(OH).sub.2 fed to the system/moles of SO.sub.2 in the feed gas). In one pilot plant, increasing the recycle ration (g solids recycled/g fresh Ca(OH).sub.2) from 6:1 to 12:1 increased SO.sub.2 removal in the spray dryer from 70% to 80% at stoichiometry 1.0 (Blythe et al., 1983, Proceedings: Symposium or Flue Gas Desulfurization, Vol. 2, NTIS PB84-110576). In another installation, recycle tests gave 10 to 15% more SO.sub.2 removal at stoichiometry 1.5 (Jankura et al., presented at the Eighth EPA/EPRI Symposium on Flue Gas Desulfurization, New Orleans, La., 1983).
Another option enhancing lime utilization uses the recycle of both solids captured downstream in the spray dryer and solids from the baghouse. However, removal does not appear to be significantly different when either spray dryer solids or fabric filter solids are employed as the recycled material. At stoichiometry 1.0 the removal increased from 53% when no recycle was employed to 62% with 0.5:1 recycle ration. When ash content in the feed slurry increased from 5% to 20%, SO.sub.2 removal in the spray dryer increased from 80% to 92% for stoichiometry 1.6 (Jankura et al., 1983).
U.S. Pat. No. 4,279,873, to Felsvang et al., relates several experiments investigating the effects of fly ash recycle and proved it to be beneficial for SO.sub.2 removal in a spray dryer. It was found that substantially higher removal of SO.sub.2 may be achieved when recycling the fly ash and Ca(OH).sub.2 than when recycling Ca(OH).sub.2 alone. Corresponding efficiencies for stoichiometry 1.4, 500 ppm inlet SO.sub.2, and comparable solids concentration were 84% and 76%, respectively. For the same stoichiometry and SO.sub.2 concentration, removal was only 67% for the simple once-thru process. At low SO.sub.2 concentration and high recycle ratios, over 90% removal was achieved even at extremely low stoichiometries. At 548 ppm SO.sub.2, 25:1 recycle, 0.76 stoichiometry and at 170 ppm SO.sub.2, 110:1 recycle, 0.39 stoichiometry, SO.sub.2 removal was 93.8% and 97.8%, respectively.
Removal efficiencies up to 65% were reported with a slurry of highly alkaline (20% CaO) fly ash only (Hurst and Bielawski, Proceedings: Symposium on FGD, EPA-600/981-019b, 853-860, 1980). In another experiment, 25% SO.sub.2 removal was achieved when spraying slurried fly ash collected from a boiler burning 3.1% sulfur coal (Yeh et al., Proceedings: Symposium on Flue Gas Desulfurization, EPRI CS-2897, 821-840, 1983). A weak trend was found in a study of 22 samples of fly ashes that a slurry with a higher total slurry alkalinity tended to have a higher SO.sub.2 capture (Reed et al., Environ. Sci. Technol., 18, 548-552, 1984).
Flue gas desulfurization by dry injection of a calcium-based sorbent such as lime into the flue gas downstream of economizer or air preheater has been a very attractive concept because of its technical simplicity and low capital cost requirement. In the dry injection procedures known in the art, lime powder, when used as sorbent, has to be very dry (i.e., containing less than about 5% water by weight) to stay in the free-flowing state for solids handling and dry injection. If the moisture content of the lime is increased above this level, it becomes a wet, sticky lime having a tendency to cake and cause soaling and plugging problems. However, the dry lime sorbent is not reactive toward SO.sub.2 unless the surface moisture is increased. Thus, for effective SO.sub.2 removal using a dry lime sorbent, the flue gas must typically be humidified by evaporation of atomized water droplets, thereby increasing the moisture content and the sorbent reactivity.
A large vessel may be required to provide adequate mixing and residence time to achieve sufficient flue gas humidification. Without such a vessel, water evaporation would be limited and the resulting duct scaling and plugging would present problems in operation. It should be noted that even after the flue gas is adequately humidified for efficient SO.sub.2 removal (to an approach to saturation temperature of 5.degree. to 30.degree. C.), the reactivity of injected lime is still low. Usually less than 20% utilization of the lime injected as expected, thereby wasting more than 80% of the sorbent. Therefore, there is a need for a reactive sorbent which can be handled by conventional dry solids injection equipment and yield high utilization.
Calcium silicate hydrates have heretofore presented problems when used as a sorbent for spray drying methods of flue gas desulfurization. As a relatively high humidity (5.degree. to 30.degree. C. approach to saturation temperature) has to be maintained in the spray dryer for effective desulfurization, and the hydrates cannot be adequately dried at high humidity (probably due to their water retention characteristics and agglomerating tendencies when slurried), operating problems such as wet deposition and scaling in the spray dryer vessel typically occur.
Calcium silicate hydrates have been shown to be a reactive sorbent for SO.sub.2 removal when injected dry into a pre-humidified flue gas stream. However, the sorbent preparation procedure for dry injection involves drying a calcium silicate slurry to produce the dry sorbent, which is a very energy intensive process. Therefore, there is a need to improve the sorbent preparation process to reduce or eliminate the drying requirement and decrease the production cost.
In the foregoing discussion, emphasis has been upon SO.sub.2 removal for several reasons: (1) sulfur oxides are one of the more difficult acid gases to control, and (2) sulfur oxide emissions nationwide are substantially greater than other acid gases. Generally, in order of decreasing reactivity toward alkali materials, the acid gases (compounds which exhibit acid behavior when mixed with water) are (HF/HBr/HCl SO.sub.3), SO.sub.2, CO.sub.2, NO.sub.2. Thus, it is believed that for any acid gases which are more reactive than SO.sub.2, the potential for significant control by calcium silicates is at least as great as for SO.sub.2. Hence, the remaining discussions will refer to acid gases meaning any individual gas, or combination of gases, which are as reactive or more reactive than SO.sub.2.